1. Field of the Invention
The present invention relates to novel naphthalenetetracarboxylic acid diimide derivatives which are useful for an organic electronics material, particularly for an organic photoconductor material.
The present invention also relates to electrophotographic photoconductors including a photoconductive layer containing at least one of the naphthalenetetracarboxylic acid diimide derivatives.
2. Description of the Related Art
Organic electrophotographic photoconductors, which have been widely put into practice, contain an organic photoconductor material. The organic photoconductor material has been required to meet more strict requirements; e.g., higher sensitivity and applicability to environmentally sensitive production methods or processes.
The organic electrophotographic photoconductor is formed from a charge generation material and a charge transport material. The charge transport material is required to have higher sensitivity; i.e., to exhibit high charge transferrability (carrier transferrability). Currently, hole transport materials, which exhibit practical applicability, are employed in many cases.
Nevertheless, further advancement of organic electronics materials (e.g., organic photoconductor materials) depends on development of a charge transport material having high performance for transporting both holes and electrons (i.e., carriers). In view of this, demand has arisen for an electron transport material exhibiting high charge transferrability (carrier transferrability).
In recent years, information processing devices based on electrophotography have been remarkably developing. In particular, laser printers and digital copiers, which convert information to digital signals and record it using light beams, have been drastically improving in reliability and printing image quality. Further, high-speed technologies allow some of the printers and copiers to achieve full-color printing. Thus, photoconductors are particularly required to attain both high image quality and high durability.
The photoconductors used in the electorophotographic laser printers, digital copiers, etc. are generally formed of an organic photoconductor (OPC) material, from the viewpoints of reducing costs, enhancing productivity, and avoiding environmental pollution. In general, the OPC material-containing photoconductor has a single-layer structure or a functionally-separated multi-layer structure. A PVK-TNF charge-transfer complex photoconductor, which is the first practical OPC-photoconductor, has a single-layer structure.
In 1968, Hayashi and Regensburger independently invented a PVK/a-Se multi-layer photoconductor. Melz et al. (in 1977) and Schlosser (in 1978) reported a multi-layer photoconductor having an organic pigment-dispersed layer and an organic low-molecular-weight compound-dispersed polymer layer; i.e., having only organic photoconductive layers. These multi-layer photoconductors are called a functionally-separated multi-layer photoconductor, since they contain a charge generation layer (CGL), where charges are generated upon absorbing light; and a charge transport layer (CTL), through which the generated charges are transported to neutralize the surface charges.
Organic photoconductors suffer from greater film ablation caused by repetitive use, as compared with inorganic photoconductors. Ablation of the photoconductive layer, which constitutes the photoconductor, leads to reduction in charge potential and deterioration of photosensitivity. In addition, background smears are easily generated by the irregularities of the photoconductor surface, and the formed image density or quality tends to be deteriorated. Thus, conventionally, organic photoconductors have been required to have sufficient wear resistance. Furthermore, since photoconductors become smaller in accordance with increased high-speed performance or downsizing of electrophotographic apparatus, much importance is placed on providing highly durable organic photoconductors.
As has been widely known, wear resistance of photoconductors is increased by curing the photoconductive layer, by imparting lubricity thereto, by incorporating a filler thereinto, or by using a charge transport polymer instead of a low-molecular-weight charge transport material (CTM)-dispersed polymer layer. However, problems newly arise by employing the above method for preventing the photoconductive layer from ablation. Specifically, on a surface of the photoconductive layer are adsorbed ozone, NOx and oxidation substances generated as a result of repetitive use or in response to ambient environment. The surface may be reduced in resistivity after repetitive use or in response to the working environment, causing problematic image blur. This image blur has conventionally been improved in some degree by ablation, together with the photoconductive layer, of these image-blur-causing substances.
However, new countermeasures must be taken for meeting the recent requirements; i.e., higher resolution and higher durability. In one proposed method for reducing the adverse effects of the image-blur-causing substances, the photoconductor is provided with a heater. However, provision of the heater is a grave obstacle to downsizing of an electrophotographic apparatus or to reduction of power consumption. Also, use of additives (e.g., an antioxidant) is advantageous. However, since usual additives have no photoconductivity, the photoconductive layer containing a large amount of additives exhibits deteriorated electrophotographic properties such as reduced sensitivity and increased residual potential.
As described above, electrophotographic photoconductors exhibit a reduced degree of ablation with a result of changing the process design therearound or imparting high wear resistance thereto, but these unavoidably give adverse side effects to image quality; e.g., image blur and reduction of resolution. Thus, difficulty has been encountered in fabricating electrophotographic photoconductors having both high durability and high image quality. In other words, they preferably have higher resistivity for preventing image blur, and in contrast, they preferably have lower resistivity for preventing increase in residual potential, which causes trade-off therebetween.
Most practical electrophotographic photoconductors have a functionally-separated multi-layer structure, which includes a conductive support, a charge generation layer, and a charge transport layer containing hole transport materials, with the layers being provided on the support. These electrophotographic photoconductors are generally used in negative electrophotographic processes.
In the electrophotographic processes, corona charging is reliably used for charging. Most copiers and printers employ this charging method. Nevertheless, as has been known well, negative corona charging is unstable compared with positive corona charging. Thus, a scorotron charger is employed, which leads to cost elevation. In addition, negative corona charging generates a larger amount of ozone which may give chemical damage to photoconductors. Thus, when the devices employing negative corona charging are used for a long period of time, the generated ozone during charging deteriorates through oxidation binder resin and charge transfer materials. Furthermore, ionic compounds (e.g., nitrogen oxide ions, sulfur oxide ions and ammonium ions) are also generated during charging, accumulation of such ionic compounds on the photoconductor causes problematic deterioration of image quality. Meanwhile, ozone is also an environmental pollutant and thus, generation of a large amount thereof is problematic. In view of this, for the purpose of preventing ozone from being released outside, ozone filters are often used in negatively-charged copiers or printers, which also causes cost elevation in these devices.
In order to solve the above-described problems, development of positively-charged type electrophotographic photoconductors has been continued. Employment of positively charging reduces generation of ozone, nitrogen oxide ions, etc. Furthermore, when widely-used two-component developers are employed, the positively-charged electrophotographic photoconductors are less affected by environmental factors, and can consistently provide desired images. Thus, the positively-charged electrophotographic photoconductor is preferred from the viewpoints as described above.
In the positively-charged single-layer photoconductor or reverse multi-layer photoconductor (CGL/CTL), charge generation materials are present in the vicinity of the photoconductor surface. The charge generation materials are very susceptible to oxidation substances such as ozone and nitrogen oxide ions and thus, the photoconductors of this type pose a problem in that they are highly susceptible to gas contained in the working environment (e.g., discharge gas from fan heaters or vehicles).
In high-speed copying processes, negatively-charged photoconductors are more preferable than positively-charged photoconductors. This is because, among existing organic materials, only hole transport materials (i.e., materials transferring only holes) exhibit such high charge mobility that is applicable to the high-speed copying processes, and normal multi-layer electrophotographic photoconductors (CTL/CGL) are only negatively charged.
As described above, electrophotographic photoconductors that can be both positively and negatively charged can be used in a wide variety of applications. In addition, cost reduction can be realized by decreasing the number of photoconductor products. Furthermore, they can be advantageously employed in high-speed processes.
Under such circumstances, Japanese Patent (JP-B) No. 2732697 discloses an electrophotographic photoconductor which can be both positively and negatively charged. This electrophotographic photoconductor contains a diphenoquinone derivative serving as an electron transport material. The diphenoquinone derivative has rather low charge mobility and thus, the electrophotographic photoconductor does not exhibit such sensitivity that is sufficiently applicable to high-speed, downsized copiers or printers. Furthermore, it causes problematic image blur as a result of repetitive use.
Japanese Patent Application Laid-Open (JP-A) No. 2000-231204 discloses a dialkylamino group-containing aromatic compound which is incorporated into photoconductors as an acid-neutralizing agent. The photoconductors containing this compound maintain satisfactory image quality even when repeatedly used. However, this compound has low charge transferrability and thus, the photoconductors are difficult to meet high-sensitive, high-speed requirements, which imposes limitation on the compound content.
JP-A No. 60-196768, JP-B No. 2884353, and other literature disclose a dialkylamino group-containing stilbene compounds. From the literature “Itami et al., KONICA Technical Report, Vol. 13, p. 37, 2000”, these stilbene compounds generate anti-oxidation gas and prevent image blur.
These compounds have a dialkylamino group as a substituent exhibiting a strong mesomeric effect (+M effect) at a resonance site of the triarylamine structure serving as a charge transporting site. Thus, the compounds exhibit extremely low ionization potential. The photoconductive layer containing, as a hole transport material, only the compound exhibits considerably deteriorated charge retention capability from the beginning of use or after repetitive use. The electrophotographic photoconductors having such a photoconductive layer pose a critical problem in that they are very difficult to put into practice. Even when used in combination with another charge transport material, the stilbene compound, in many cases, has ionization potential considerably lower than that of the material, making hole-trap sites for charge transfer. Thus, the formed electrophotographic photoconductors exhibit drastically reduced sensitivity and high residual potential.
JP-A No. 2004-258253 discloses a photoconductor containing a stilbene compound and a specific diamine compound. The photoconductor can be repeatedly used without reducing sensitivity thereof and exhibits improved environmental stability against, for example, oxidation gas.
The photoconductor, however, has not yet realized high-speed printing and downsizing of an electrophotographic apparatus.
Naphthalenetetracarboxylic acid diimide derivatives are reported to be used as an acceptor for forming a charge transfer complexes (see, for example, Germany Patent No. 1230031 (AUSLEGESCHRIFT 1230031)); or are reported to be used as a compound (pharmaceutical drug) exhibiting anti-helicobacter activity (see International Publication No. 02/040479). However, in the above literature, they are not suggested to be used in electrophotographic photoconductors. Separately, U.S. Pat. No. 5,468,583 describes use, as an electron transport material, of bis-N-alkyl-naphthalenetetracarboxylic acid diimide derivatives having a different structure from naphthalenetetracarboxylic acid diimide derivatives of the present invention. Electrophotographic photoconductors containing the bis-N-alkyl-naphthalenetetracarboxylic acid diimide derivative pose a problem in that they cause image blur after repetitive use.
The aforementioned JP-B No. 2732697 suggests use of diphenoquinone derivatives as an electron transport material used in electrophotographic photoconductors. JP-A No. 2005-154409 and U.S. Pat. No. 6,794,102 disclose naphthalenetetracarboxylic acid diimide derivatives. The literature “Chemistry letters (2003), 32(6), 508-509”describes semiconductive materials formed of a naphthalenetetracarboxylic acid diimide derivative having the following structural formula (1). The International Publication No. 00/040657 describes electron transport materials formed of naphthalenetetracarboxylic acid diimide derivatives having the following structural formulas (2) and (3).

As has already been described above, the diphenoquinone derivative described in JP-B No. 2732697 has rather low charge mobility and thus, the photoconductors containing it do not exhibit such sensitivity that is sufficiently applicable to high-speed, downsized copiers or printers. Furthermore, this derivative causes problematic image blur as a result of repetitive use. The diphenoquinone derivatives described in JP-B No. 2732697, the naphthalenetetracarboxylic acid diimide derivative (2) or (3) described in the International Publication No. 00/040657, and diphenoquinone derivatives described in JP-A No. 01-206349 have poor compatibility to binder resin forming photoconductors, failing to sufficiently exhibit their characteristics. The naphthalenetetracarboxylic acid diimide derivative described in JP-A No. 2005-154409 and U.S. Pat. No. 6,794,102 have a complicated structure, involving problems in production thereof The naphthalenetetracarboxylic acid diimide derivative (1) described in the literature “Chemistry letters (2003), 32(6), 508-509” has low electron transferrability, not being applicable to practical use.